1. Field of the Invention
This invention relates to electro-optic sensors that provide both imaging and range sensing.
2. Description of the Related Art
Electro-optic (EO) sensors detect optical radiation emanating from one or more objects in a scene to form an image of those objects. The optical radiation may be emitted from or reflected off of the objects. EO sensors span an electromagnetic band including UV, visible, NIR, SWIR, MWIR and LWIR spectrums. The images are typically processed to estimate a line-of-sight (LOS) angle to one or more objects. The image may also be processed to extract other information about the object in order to discriminate or classify objects.
In certain applications it would be useful to augment the standard imaging function with accurate range information. The ranging function may be provided by an auxiliary active optical target detector such as a laser rangefinder. The laser reflects an optical pulse off the of the object and a detector determines the time of flight to estimate the range. Although effective, the active system is costly. Another approach is to use multiple apertures to make parallax measurements to provide the range estimate. Although passive this approach suffers from degraded accuracy and a difficult calibration process.
Another approach, commonly referred to as “coded aperture” uses a patterned aperture stop to modulate the wavefront to form an image having distinct spatial frequency components at the detector. Accurate range information can be extracted from the structure of the image (usually the spatial frequency components of the image are compared). However, the range estimates are typically only accurate over a 4× dynamic range. An image of an object (a local region of the scene) can be reconstructed from the image of the modulated waveform. The imaging function does suffer from an approximately 50% loss due to the patterned aperture stop and noise associated with the reconstruction process. Different spatial spatterns (“codes”) will perform better for the ranging function or the imaging function. Typically, a spatial pattern is selected that provides an adequate balance of the two functions. A complete explanation of the technique for simultaneous imaging and passive range detection using “coded aperture” is provided in the following references: Edward Dowski and W. Thomas Cathey “Single-lens single-image incoherent passive-ranging systems” APPLIED OPTICS Vol. 33, No. 29, pp. 6762-6773 10 Oct. 1994; Levin, Anat, et al. “Image and depth from a conventional camera with a coded aperture.” ACM Transactions on Graphics (TOG) 26.3 (2007): 70 and Gregory Johnson et al. “Passive ranging through wave-front coding: information and application” APPLIED OPTICS Vol. 39, no. 11, pp. 1700-1710 10 Apr. 2000, which are hereby incorporated by reference.